1. Technical Field of the Invention
The present invention relates to a polyester resin decreased in the amount of an organic gas generated therefrom during molding thereof and during use thereof at high temperatures, and a process for preparing the same.
2. Description of Related Art
Polyester resins such as copolymers comprising a polybutylene terephthalate or polybutylene naphthalate and further a polytetrahydrofuran and adipic acid as the constituent units are used as materials for electric components and automotive parts by taking advantage of excellent properties of them. Particularly, in recent years, they have come to be used under severe conditions including a high temperature and a high humidity. When an ordinary polyester resin is used for such a purpose, it is easily hydrolyzed or pyrolyzed to generate a gas. Particularly when the resin is positioned near an electric contact, problems are posed that, for example, the generated gas is carbonized by the arc discharge to form a deposit near the electric contact to cause an abnormal conduction at the contact. Since this problem is serious particularly in a low-molecular material excellent in fluidity, the compatibility of a high fluidity with a decreased amount of the generated gas with each other has been a difficult problem. For solving these problems, for example, there were proposed a process wherein the amount of the carboxyl terminal group is reduced and, in particular, a process wherein solid phase polymerization is conducted, particularly, a process wherein solid phase polymerization is conducted after copolymerizing a polyfunctional compound and a monofunctional compound in a specified ratio (JP-A-4-272920).
However, in the process wherein the amount of the carboxyl terminal group is reduced, the decrease in the amount of the generated gas is yet insufficient; and in the process wherein solid phase polymerization is conducted, a problem that the fluidity of the resin is lost to make the formation of a thin molding difficult is caused. When solid phase polymerization is conducted after copolymerizing a polyfunctional compound and a monofunctional compound, the Theological characteristics of the resin are changed to increase the viscosity thereof under a low shearing condition and, therefore, such a process has the problem of a brittleness lower than that of a (co)polymer having the same fluidity but prepared by other processes.